1. Field of the Invention
The present invention relates to injection molding machines, and more particularly, to a pressure controller for an injection molding machine.
2. Description of the Related Art
An in-line screw injection molding machine performs a measurement step, an injection step, and a dwell step. In the measurement step, a screw in a heating cylinder is rotated while the screw is moved backward to press and send a molten resin to a tip portion of the heating cylinder, and then, in the injection step and the dwell step, the screw is moved forward to fill a mold with the molten resin.
In the injection step, the screw is moved forward based on an injection stroke and an injection speed set in advance to fill the mold with the molten resin in the heating cylinder. When the screw is moved to an injection/dwell switching position, the injection step is switched to the dwell step. In the dwell step, pressure control is performed based on a dwell pressure and a time period set in advance to fully fill the mold with the molten resin and also to compensate for sink marks on molded articles due to shrinkage of the resin.
In thin-wall molding, a high peak pressure is produced when the resin is injected into a cavity at a high speed to fill the cavity in a short time. If the high peak pressure is not reduced immediately, stress deformation may occur in the molded article, and may result in a defective product. In a known art, a pressure from a resin to a screw is detected by a load cell in the dwell step, and the pressure is controlled to become a set pressure.
In the known art, although the control for reducing the pressure on the screw can be performed, the pressure in the mold is not always the same as the pressure to the screw. Consequently, it is difficult to reduce the pressure in the mold to a desired pressure at a desired response speed.
FIG. 8 is a graph showing time transition of injection pressure Q1 and pressure Q2 in a mold in one molding cycle in the molding processing by an injection molding machine including a pressure controller according to a known art. FIG. 8 shows a problem that the injection pressure Q1 is reduced, but the pressure Q2 in the mold is not completely reduced. In the graph, reference numeral P indicates a screw position.
Japanese Unexamined Patent Application Publication No. 62-261419 discloses a technique for controlling a pressure detected by a pressure sensor provided in a mold to become a predetermined pressure. However, it is necessary to provide the additional pressure sensor in the mold, and this increases the cost.
Japanese Unexamined Patent Application Publication No. 2000-167892 discloses a technique of providing a step for moving a screw backward by speed or position control in the latter half of a filling step, and after the completion of the filling step, performing pressure control in a dwell step.
Japanese Unexamined Patent Application Publication No. 2001-277322 discloses a technique in which, in a filing step in injection molding, a screw is moved forward to a predetermined position and moved back to a set position at a set speed for depressurization just before V-P switching. However, all of the above techniques describe to simply move the screw backward by the position control for depressurization in the latter half of the filling step, and there is a problem that it is difficult to reduce the pressure in the mold at a desired response speed.
Japanese Unexamined Patent Application Publication No. 10-235704 discloses a technique for storing a value of driving pressure of an injection screw detected in injection of a molten material in a mold-open state. Based on a difference between a driving pressure of the injection screw detected in real time during the molten material injection in a normal filling state in which the mold is closed and the driving pressure of the injection screw stored in advance, a pressure in the mold in the normal filling operation is estimated. However, injecting the molten material in the mold-open state may damage the mold.
Japanese Unexamined Patent Application Publication No. 2001-18271 discloses a technique for applying a dwell pressure based on a dwell pressure target value and detecting a resin pressure at a gate side and a resin pressure at an opposite side of the gate in a mold cavity in a dwell step. If the resin pressure at the opposite side of the gate starts to decrease, the pressure is controlled such that the resin pressure at the gate side becomes the same resin pressure at the opposite side of the gate. However, it is necessary to provide an additional pressure sensor in the mold, and this increases the cost. Further, there is a possibility that the pressure control for controlling the resin pressure at the gate side to become the same pressure as the resin pressure at the opposite side of the gate is not always suitable for the thin-wall molding.
Japanese Unexamined Patent Application Publication No. 8-244078 provides a first pressure sensor for detecting a resin reaction force acting on a screw or a pressure in a cylinder and a second pressure sensor for detecting a pressure in an injection mold. Until the resin reaches the second pressure sensor, feedback control of the injection pressure is performed based on a set resin reaction force or a set pressure in the cylinder and the current pressure detected by the first pressure sensor. After the resin reaches the second pressure sensor, the feedback control of the injection pressure is performed based on a set pressure in the injection mold and the current pressure detected by the second pressure sensor. However, it is necessary to provide the additional pressure sensors in the mold, and this increases the cost.